Its not available in T-mobile stores, only via their website. And if you visit their plans page, its pretty hidden. Scroll down to the bottom and find “$30/month – Unlimited web and text with 100 minutes talk”.

Activate online. Just make sure you choose the correct plan – T-mobile has more than one $30/month plan, but only the “100 minutes, unlimited text, 5GB web” is suitable for most users. The other plans offer very low amounts of data.

Need more than 100 minutes/month? You can prepay $100 for 1000 additional minutes that can be used within 12 months. This way, you’ll only pay $0.10/minute should you need more than 100/month.